1. Field of the Invention
The present invention relates to an automatic exposure control device for motion picture camera and other devices and particularly to an improvement of the automatic exposure control device in which a diaphragm driving arrangement keeps a diaphragm arrangement at a position corresponding to the smallest or the largest opening when the camera is not operating, and when a current is supplied to the device an electrical diaphragm presetting arrangement operates before camera release to automatically preset the diaphragm arrangement at an intermediate opening position between the largest and the smallest opening positions, and when the camera is released an exposure control device operates so as to adjust the diaphragm means to an opening for providing a proper exposure.
2. Description of the Prior Art
Most of the recent motion picture camera is equipped with an automatic exposure control device, and, therefore, able to obtain pictures under proper exposure easily. Hereby, this kind of automatic exposure control device is usually operated by the camera release operation in order to simplify the camera handling. Namely, together with the switch for controlling the current supply to the camera driving motor for driving the shutter mechanism, the film feeding mechanism and so on, the switch for controlling the current supply to the automatic exposure control device is operatively associated with the camera release button in order to simplify the camera handling. In case of the camera so designed that the switch for the automatic exposure control device and the switch for the camera driving motor are controlled by means of the camera release button, it is usual that the switch for controlling the current supply to the automatic exposure control device is actuated with the first stroke of the release button and then the switch for controlling the current supply to the camera driving motor is actuated with the second stroke.
The reason why an interval is provided between the time at which the switch for the automatic control device is actuated and the time at which the switch for the automatic control device is actuated is mainly as follows. Namely in case of this kind of the automatic exposure control device, as diaphragm driving means for driving the diaphragm in response to the output of the exposure control means for determining a proper aperture value on the basis of the output of the light sensing element and other exposure factors such as the film sensitivity information and so on, an exposure meter is mainly used. Hereby, as is usually known, this kind of exposure meter is influenced by the force of a spring, so that when the device is out of operation, the diaphragm means is held at the smallest or the largest opening position. Consequently, if the switch for the automatic exposure control device and that for the camera driving motor are actuated completely at the same time, the photographing is started before the diaphragm means is adjusted at an opening position for giving a proper exposure in accordance with the output of the exposure control means, so that the initial scene of the photography is under-exposed quite badly. Consequently, at the time of camera release, the switch for the automatic exposure control device should be actuated, before the switch for the camera driving motor is actuated, so that the photographing is started after the diaphragm means has been adjusted at an opening position for giving a proper exposure in accordance with the output of the exposure control means, so as to solve the problem of improper exposure at the initial scene.
Nevertheless, in case of this kind of camera, the above mentioned problem of improper exposure at the initial scene becomes remarkable at times. Namely, in case of this kind of camera, when the release button is operated comparatively slowly, a sufficient time during which the diaphragm means is adjusted at the opening position for giving a proper exposure can be obtained and, therefore, there takes place no problem. On the other hand, when the release button is operated abruptly, the time during which the diaphragm means is adjusted at the opening position for giving a proper exposure can hardly be obtained, so that the photographing is started before the diaphragm means has been adjusted at the opening position for giving a proper exposure, whereby the initial scene is improperly exposed in the same way as in the afore mentioned case.
In view of such an inconvenience as mentioned above, an automatic exposure control device so designed that the diaphragm means is once preset at an intermediate position between the smallest and the largest opening positions at taking picture before the camera release so as to eliminate the difficulties of the improper exposure at the initial scene has been proposed. Namely, according to Japanese Patent Laid-Open Publication No. Sho 50-17223 (Title "Exposure Meter Device", published on Feb. 24, 1975) of the assignee of the present patent application, an automatic exposure control device is proposed, in which the afore mentioned exposure meter as the diaphragm driving means is connected to an exposure control circuit as exposure control means including a light sensing element for measuring the brightness of the object to be photographed and a variable resistance for inputting the film sensitivity information and a resistance as an electrical presetting means for presetting the diaphragm means at the afore mentioned opening position before the camera release, is connected to the afore mentioned exposure control circuit so as to cooperate with said variable resistance. Further, a switch is provided as a change-over means which makes a resistance circuit consisting of the afore mentioned diaphragm presetting resistance and the variable resistance for inputting the film sensitivity information connected to a power source built in the camera body, and electrically excluds the afore mentioned light sensing element in the state before the camera release, but connects the exposure control circuit consisting of the light sensing element and the variable resistance for inputting the film sensitivity information to the power source through a constant voltage circuit and electrically excluds the diaphragm presetting resistance with the first stroke when the camera is released. According to such a construction of the automatic exposure control device, when the power-source switch is closed at taking pictures, the diaphragm means is always preset at the afore mentioned intermediate opening position by the effect of the resistance circuit consisting of the afore mentioned diaphragm presetting resistance and the variable resistance for inputting the film sensitivity information before the camera is released, so that the inconvenience of the improper exposure at the initial scene can be eliminated even when the camera is released abruptly. Especially in case of this device since the diaphragm presetting resistance circuit includes the resistance for inputting the film sensitivity information, the diaphragm presetting position alters in accordance with the sensitivity of the film to be used in such a manner that when a high sensitivity film is used the diaphragm means is preset at a comparatively small opening position while when a low sensitivity film is used the diaphragm means is preset at a comparatively large opening position which is quite rational.
The following is advantageous when the system forms an open loop, but is disadvangageous when it forms a closed loop. Throughout the specification, the term "non-servo" is used to identify an open loop arrangement and "servo" is used for a feedback or closed loop arrangement. In an open loop exposure control system, light from the object strikes a light receiving element without passing a diaphragm that is controlled by the system. In a closed loop or servo arrangement, light from the object strikes the light sensing element only after passing through a diaphragm. The diaphragm may be either the diaphragm in the objective lens, or a diaphragm specifically placed in front of the censor and coupled to the objective diaphragm to open and close therewith. In the closed loop arrangement, the light sensing element in the exposure control circuit receives the light passing through the diaphragm, so that if the preset position of the diaphragm changes in accordance with the film sensitivity, the amount of light incident upon the light sensing element changes accordingly. Thereby in case a film with a comparatively low sensitivity is used, the diaphragm means is preset at a comparatively large opening position correspondingly, so that the amount of the light incident upon the light sensing element is also comparatively large, whereby there takes place no such important problem, while in case a film with a very high sensitivity is used, the diaphragm means is preset at a small opening position correspondingly, so that the amount of the light incident upon the light sensing element is also small, whereby the problem of the response delay of the light sensing element becomes especially remarkable. Namely, in such a device as mentioned above a photo-conductive element such as a CdS and so on is usually applied as the light sensing element, however, the photo-conductive element is not satisfactory in response especially for the light with low brightness, so that in case the light is of low brightness, it takes comparatively a long time for the output to reach the level corresponding to the light incident thereupon. Now let us suppose that the sensitivity of the film to be used is as high as ASA 160, while at first the diaphragm means is set at the aperture value f45 and the brightness of the object to be photographed is so low that the diaphragm means is to be set at the aperture value f1.4 in order to obtain a proper exposure. Hereby it takes such a long time as 15-30 sec. for the diaphragm means with f45 to reach f1.4. Hereby, taking it into consideration that the response delay of the diaphragm means basing upon the mechanical engagement of the diaphragm means with the exposure meter is almost negligible (for example, as short as 0.5 sec.), the above mentioned time of 15-30 sec. is almost due to the response delay of the light sensing element. Of course the above is an extreme example, however, the response of the photo-conductive element is usually inferior for the light with low brightness. Consequently, if the afore mentioned diaphragm presetting device is applied to the automatic exposure control device of the servotype in which a photo conductive element is used as light sensing element, when an object with a low brightness is photographed with a film with an especially high sensitivity the diaphragm means is preset at a position corresponding to a small opening, while further the amount of the light incident upon the light sensing element is remarkably small because of the low brightness of the object to be photographed, so that in this case the initial scene is under exposed due to the response delay of the light measuring element. As explained above, the feature that the diahragm presetting position changes in accordance with the sensitivity of the film to be used is, on the contrary, inconvenient for the device of servo-type.
Beside the above, in said proposed device, a current is supplied thereto through the constant voltage circuit whereby a power less takes place in the constant voltage circuit, which adds to the inconvenience.
Further, in the proposed device, as the change over means, so called transfer switch which is changed over from the contact connected to the diaphragm presetting resistance to the contact connected to the constant voltage circuit (to which the afore mentioned exposure control circuit is connected) in response to the camera release is used, however, in case the transfer switch is used as change over means the change over operation is carried out slowly for example when the release button is operated very slowly, so that there is a danger that before the start of the current supply to the exposure control circuit the once preset diaphragm means should be sifted, under the effect of the spring working upon the exposure meter, to the stop position in unoperative state of the device, namely the position corresponding to the smallest or the largest opening. In such a case, the problem of the improper exposure of the initial scene becomes remarkable again despite the diaphragm presetting device.